Concentration of sodium hydroxide



Patented June 5, "1934 ooncimm'non or sonnm maoxma' Robert a. Mullins, Niagara Falls, N. n, in-

either to The. Mathison Alkali Works, New York, N. Y a corporation of Virxlnia Application Septembesin, 19st, Serial No. 833,854

:1 Claims. (01. 23-184) This invention relates to improvements in the concentration of sodium hydroxide. ,More particularly, the invention relates to improvements in the production 01' concentrated aqueous solutions ci' sodium hydroxide.

It has been known for some time that sodium hydroxide monohydrate, NaOH-HzO', can be precipitated from aqueous solutions oi. sodium hydroxide initially containing upwards oi. 20% by weight NaOH at temperatures ranging from below 0 C. to over C. by the addition of ammonia. Separationoi sodium hydroxide monohydrate s0 precipitated, however, involves technical dimculties because of the ammonia concentradroxide initially containing upwards of 20% NaOH a separation oi the solution into two liquid layers the lower of which contains sodium hydroxide in higher concentration than the oriinal solution can be effected at temperatures higher than those at which sodium hydroxide mono hydrate is precipitated. This discovery .enables me. to eflect the separation-oi the more concentrated aqueous solution 01- sodium hydroxide constituting thelower liquid layer as' a separato avoid such technical diiliculties; According to my invention, ammonia is added to aqueous solutions of sodium hydroxide con taining upwards of 20% NaOl-I and the solution, to which ammonia has been added, is separated into two liquid layers the lower of which contains sodium hydroxide in higher concentration' than the original solution at an elevated 40 temperature. Temperatures as low as 35C. are useful, in some cases, but in general I find the range ot.50-"I0 C. advantageous. I find a temperature approximating C.- to be particularly advantageousi At temperatures approximating so" 0., distinctdiflerences in density promoting rapid and definite separation of the two liquid layers are developed, and without the development of excessive pressures. The ammonia may be added to the aqueous solution of sodium hyo droxide at low temperature and the solution then heated to the temperature at which separation -is eflected or .the ammonia may be added to the solution at the elevatedtemperature' at which the separation iseflected.

a The lower liquid layer contains sodium hytions necessary to eii'ect the precipitation. This applied. tion 01' one liquid irom another liquid and thus droxide in higher concentration than the original solution and also ontains some dissolved ammonia. Following segregation of the two liquid layers separated in the practice of my invention, this lower liquid layer can be purged of 60 ammonia by reduction of pressure and. heating or in some cases by reduction of pressure/alone. The upper liquid layer contains some sodium hydroxide and most of the ammonia. Following segregation of the two liquid layers, this am- 5v monia can be recovered in substantially anhyidrous form by distillation therefrom with concurrent production of a relatively dilute aqueous 1 solution of sodium hydroxide. 0r, after segregation-ot the two liquid layers, sodium hydroxide monohydrate may be precipitated from the upper liquid layer by chilling and the sodium hydroxide monohydrate thus separated added .to the segregated. lower liquid. layer further to increase the concentration of sodium hydroxide therein.

My invention is not limited in application to pure solutions of sodium hydroxide but is'also applicable to other aqueous solutions containing sodium hydroxide in concentrations upwards of 20% NaOH. In some cases, 'itls advantageous 8G toadd water soluble salts compatible with sodium hydroxide toincrease the efliciency oi the, operation; Sodium nitrate may be so added to the solution to which my invention is to be 1 When sodium nitrate is added to the aqueous solution of sodium hydroxide prior to the separa tion of the two liquid layers, in out my invention, the sodium hydroxide concentrates in the lower liquid layer and the sodium nitrate concentrates in the upper liquid layer. The sodium nitrate content oi-the upper liquid layer can be recovered, after segregation oi the two liquid layers, for example, by subjecting the upper liquid layer to the action of carbondioxide to produce sodium carbamate NaCOzNI-Iz, and ammonium nitrate or by crystallization iollowing evaporative concentration. The small proportion of sodium nitrate accompanying the concentrated sodium hydroxide in the lower liquid layer, it objectionable, can be separated therefrom during subsequent evaporation as sodium carbonate, sodium sulphate and sodium chloride are conventionally separated during 'evaporative concentration of caustic soda liquors.- For the ultimate-production 106 oi-iused anhydrous sodium hydroxide, the presence oi such relatively small proportions of so-' diumnitrate is advantageous, rendering unnecessary the conventional addition of sodium nitrate tor purification oi the fused product. 110

cantation. After separation ofammonia, by dis-1 Example-I 200 parts (by weight) of anhydrous ammonia are added to 100 parts of an aqueous solution of sodium hydroxide containing 25% by weight NaOI-I. The-mixture is brought to a. temperature of 60 C., briefly stirred and permitted to settle.

Two liquid layers separate and are segregated by decantatlon. After separation of ammonia, by

distillation, the lower layer consists of an aqueous solution of sodium hydroxide containing about by weight NaOI-I and the upper layer consists of an aqueous solution of sodium hydroxide containing about 19% by weight NaOH. The recovered ammonia may be reused in the operation.

- Example II 125 parts of anhydrous ammonia are added to 100 parts of an aqueous solution of sodium hydroxide containing 30% by weight NaOH. The mixture is brought to a temperature of {0 C., briefly stirred and permitted to settle. Two liquid layers separate and are segregated by detillation, the lower layer consists of an aqueous solution of sodium hydroxide containing about 47% by weight NaOH and the upper layer consists of an aqeous solution of. sodium hydroxide containing about 21% by weight NaOH.

Example III 150 parts of anhydrous ammonia and 30 parts of sodium nitrate are added to 100 parts of an aqueous solution of sodium hydroxide containing 30% by weight NaQH. The sodium nitrate may be supplied in solution with the ammonia. The

mixture is brought to a temperature of C.,

briefly stirred and permitted to settle. Two liquid layers separate and are segregated bydecantation.

After separation of ammonia, by distillation, the

lower layer consists of an aqueous solution of sodium hydroxide containing, about 75% of the total sodium hydroxide and about 10% of the total sodium nitrate in the form of an aqueous solution of sodium hydroxide and sodium nitrate contain- -ing more than 50% by weight NaOH.

\ It will be understood that the separation and segregation of the two liquid layers, in carrying out my invention, is effected under a superatmospheric pressure, for:example, of 75-300 pounds per square inch, in order to maintain the ammonia concentration in the reaction mixture. The concentrated aqueous solutions of sodium hydroxide produced by my invention, separated as the lower liquid layer, may be used as such or, for example, they may be subjected to further oper;

ations for further concentration. These concemtrated solutions may be evaporated for the ultimate production of fused anhydrous sodium hydroxide or sodium hydroxide monohydrate may be precipitated from these. solutions by chilling. Similarly, the relatively dilute solutions 01 0- dium hydroxide produced in the practice of my invention, separated as the upper liquid layer, may be used as such or they may be subjected to further treatm'nt for the recovery of contained sodium hydrox de, and any contained salts such assodium nitrate.

My invention can be embodied in various combined operations involving, as one step, the concentration of aqueous'solutions of sodium hydroxide. For example, an operation of the type illustrated by Example III can be embodied in a combined operation for the production of aqueous solutions of sodium hydroxide containing upwards of 50% NaOH as follows:

a Ammonia and sodium nitrate are added to an aqueous solution .of sodium hydroxide containing upwards of 20% by weight NaOH, this mixture is separated into two liquid layers and the two liquid layers are segregated, as in Example III. The upper liquid layer is carbonated to precipitate sodium carbamate, for example as described in an application filed June 4, 1932, Serial Number 615,464, by Wesley K. McCready and me. The precipitated sodium carbamate is separated from the liquor, by filtration for example, and is then causticized' withlime in the presence of water to produce an aqueous solution of sodium hydroxide containing upwards of 20% NaOH which is supplied to the first-mentioned step. The liquor from which the sodium carbamate has been separated is stripped of ammonia, by distillation, for example, and is then treated with sodium chloride to convert the ammonium nitrate content of this liquor to sodium nitratewith precipitation of ammonium chloride. After separation of the precipitated ammonium chloride, by filtration for example, this sodium nitrate solution is supplied to the first-mentioned step directly or it may be supplied to that step indirectly by being supplied to the causticization of the sodium carbamate with lime. The lower liquid layer is stripped of ammonia, by distillation for example, and then constitutes the product of the operation, the concentrated aqueous solution of sodi hydroxide. The ammonia recovered in each 0 e two stripping operations mentioned is returned to the first-mentioned step.

I claim:

1. In the production of concentrated aqueous solutions of sodium hydroxide, the improvement which comprises adding ammonia to an aqueous solution of sodium hydroxide containing upwards of 20% by weight .NaOH and separating the solution into two liquid layers the lower of which contains sodium hydroxide in higher concentration than the original solution at a temperature upwards of 35 C.

2. In the production of concentrated aqueous solutions of sodium hydroxide, the improvement which comprises adding ammonia to an aqueous solution of sodium hydroxide containing upwards of 20 95 by weight NaOH and separating the solution into two liquid layers the lower of which contains sodium hydroxide in higher-concentration than the original solution at a temperature of 50-'10 C.

3. In the production of concentrated aqueous solutions of sodium hydroxide, the improvement which comprises adding ammonia to an aqueous solution of sodium hydroxide containing upwards of 20% by weight NaOH and separating the solution into two liquid layers the lower of which contains sodium hydroxideun higher concentration than the original solution at a temperature approsimstins 60 C.

4. In the production of concentrated aqueous solutions of sodium hydroxide, the improvement tion into two liquid layersthe lower of which contains sodium hydroxide in higher concentration than the original solution at a temperature higher than that at which sodiumhydroxide monohydrate is precipitated, and purging the lower liquid layer of ammonia alter segregation from the upper liquid layer. v

6. In the production of concentrated aqueous solutions of sodium hydroxide, the improvement which comprises adding ammonia to an aqueous solution of sodium hydroxide containing upwards of 20% by weight NaOH and separating the.solution in the presence oi'a sodium salt relatively solubl in both water-and liquid ammonia and relative y insoluble in concentrated aqueous solutions of sodium hydroxide into two liquid layers the lower of which contains sodium hydroxide in higher concentration than the original solution at a temperature higher than that at which sodium hydroxide monohydrate is precipitated.

7. In the production of concentrated aqueous solutions of sodium hydroxide, the improvement which comprises adding ammoniato an aqueous solution of sodium hydroxide containing upwards of 20% by weight NaOH and separating the solution in the presence of a sodium salt relatively soluble-in both water and liquid ammonia and relatively insoluble in concentrated aqueous solutions of sodium hydroxide into two liquid layers the lower of which contains sodium hydroxide in higher concentration than the original solution at a temperature upwards of 35C.

' ROBERT B. MACMULLIN. 

